The invention is directed to a measuring circuit for a capacitive sensor for distance measurement and/or space monitoring comprising a sensor wire and a shielding electrode.
In recent times, capacitive sensors have repeatedly proven themselves successful in space monitoring and also in measuring distances from obstacles, for example, in the vicinity of motor vehicles. A method known from DE 195 01 642 A1 for contactless distance measurement which is used as a parking aid employs ultrasonic sensors and also a capacitive sensor, although its measuring circuit is often insufficient for stricter requirements respecting accuracy.
Therefore, it is the object of the present invention to provide a measuring circuit of the type mentioned above, by means of which it is possible to measure input current and capacitance sensitively and with high resolution.
According to the invention, this object is met in that a sine signal is applied to the shielding electrode, in that the sensor wire is connected, via a shielded cable, with one input of an input amplifier which serves as current-voltage converter and whose supply voltage is likewise influenced by the sine signal, in that the output of the input amplifier is connected with one input of a phase-dependent rectifier arrangement, the sine signal is applied to the other input of the phase-dependent rectifier arrangement and its output is connected to an analog-to-digital converter.
The measuring circuit according to the invention has the advantage that it requires few structural component parts, is not susceptible to temperatures, has an extremely low input capacitance (less than 10 fF) and has a measuring speed of less than 100 xcexcs. A further advantage consists in that this measuring circuit is appropriate for motor vehicles because it does not cause interference particularly in the long wave range of car radios.
An advantageous construction of the measuring circuit according to the invention consists in that the input amplifier is a negative feedback differential amplifier for a capacitively induced differential current whose inverting input is connected with the sensor wire and whose noninverting input is connected to a voltage divider which is connected between a constant voltage source and the sine signal source, wherein the resistor of the voltage divider connected with the sine signal source is connected in parallel with a capacitor.
Because of the negative feedback and the resulting low input resistance, the input capacitances and coupling capacitances hardly impair sensitivity. Further, the negative feedback reduces the effective input capacitance by the amount of the no-load voltage gain of the amplifier. Accordingly, changes in capacitance in the environment which are several orders of magnitude below the input capacitance of the circuit can be reliably detected.
In this respect, it has proven particularly successful when the differential amplifier is an operational amplifier whose supply voltage is stabilized by a voltage regulator relative to the voltage of the shielding electrode. This prevents its limited suppression of signals on the supply voltage from simulating an input signal. Accordingly, full sensitivity can be utilized.
In order to prevent disturbances it has proven advantageous when the shielding electrode of the sensor is connected with the sine signal source via the shielding of the shielded cable. Accordingly, only very little distortion occurs, so that the EMC requirements can be met.
In a particularly advantageous construction of the invention, it is suggested that a subtraction stage is provided between the input amplifier and the phase-dependent rectifier arrangement, wherein the positive input of the subtraction stage is connected with the output of the input amplifier and the sine signal is applied to the negative input of the subtraction stage. The constant signal component originating from the modulated supply voltage is removed from the output of the input amplifier by the subtraction stage.
In another advantageous construction of the invention, it is proposed that a filter is provided at the output of the subtraction stage, which filter is used as a preliminary filter for the subsequent stage, that is, for the phase-dependent rectifier arrangement.